Infrared analyzing devices have been used to identify and analyze the concentrations of components of samples based on their absorbance at particular wavelengths. For example, the starch, protein, lipid and fiber concentrations of grains, the octane number of gasoline, the moisture content of chemicals or foods, or the lignin content of pulp and paper, can be determined through infrared analysis. While most analyzing systems are adapted for use in a laboratory, the on-site and in-line monitoring of industrial processes is an important application of this technology. In in-line monitoring, the samples are usually moving and may be inhomogeneous. Their composition and optical characteristics may change suddenly. Measurements need to be performed rapidly and a large number of such measurements may need to be averaged to correct for discontinuities in the sample or other sources of error. While a flowing sample may be stopped in a sample cell long enough to test, such a sample may not be representative of the bulk of the product stream and the testing process may be too slow for most control monitoring purposes.
Noise is a common problem effecting the accuracy of absorbance measurements. Common sources of noise are fluctuations in the emission of the radiation source, heat, degradation of system components, power fluctuations and other random variations due to internal or external causes. To correct for noise, measurements of a sample and a reference are taken and compared. If the measurements are made close enough in time, noise which effects the sample reading will also effect the reference reading and can be cancelled.
Of the two main types of analysis devices, the dual beam and single beam, only the dual beam device can simultaneously measure the sample and reference. It does this by splitting the incident radiation with a mirror, for example, and directing one beam to the sample and the other to the reference. In a single beam device the incident radiation is alternatively applied to the sample and reference by a rapidly switching mirror. Since the reference should indicate the signal level to be compared with the sample measurement, the faster the switching the more accurate the correction.
Because of their better error correction, dual beam systems are more precise than single beam systems. They require complex optics systems, however, and are therefore more expensive. The single beam system is simpler and easier to use and maintain, but is less precise. The precision of the single beam system can be improved by increasing the switching rate of the mirror, but this adds to the complexity and cost of the system. In U.S. Pat. No. 4,236,076, to Judge and assigned to Alfa-Laval AB, the precision of a single beam system is improved without adding complexity to the system through a unique averaging technique.
To determine the absorbance of a sample at a specific wavelength, the wide-band radiation emitted by a source, such as a tungsten-halogen lamp, needs to be filtered. Various mechanical methods have been employed to switch between the many filters required to shift from wavelength to wavelength during an analysis. One method is to mount a series of filters on a rotating turret or a paddle wheel. See, for example, U.S. Pat. Nos. 4,236,076 and 4,082,464. Since the sequential change in wavelength is limited by the speed of the shifting filters, analysis is too slow for in-line concentration monitoring in many manufacturing or production processes.
To increase the speed of analysis, infrared emitting diodes ("IREDs") controlled by a microprocessor have been utilized. See, for example, U.S. Pat. No. 4,401,642. Such systems can only analyze across a limited bandwidth, however, due to the limited range of IREDs (850-1050 mm). Another approach is to use vibrating holographic gratings. See, for example, U.S. Pat. No. 4,540,282. While the speed of analysis is increased, it is difficult to correct for errors.
The fastest switching between wavelengths for analysis can be achieved by an acousto-optic tunable filter, which is a crystal whose index of refraction can be altered by acoustic waves. The application of a particular frequency of acoustic wave to a birefringent crystal changes the direction of propagation and the polarization of a narrow wavelength band of the incident radiation, yielding two tuned radiation beams which diverge from each other and the non-tuned radiation. The tuned wavelength can be isolated and used to analyze a sample. The tuned wavelength band can be changed in milliseconds, depending on the speed of other components of the system. One commonly used crystal is tellurium dioxide. AOTFs are described in Harris, et al., "Acousto-Optic Tunable Filter", Journal of the Optical Society of America, Vol. 59, No. 6, pp. 744-747 (June 1969); Chang, "Noncollinear Acousto-Optic Filter With Large Angular Aperture", Applied Physics Letters, Vol. 25, No. 7, pp 370-372 (10/1977); and U.S. Pat. Nos. 3,679,288; 3,944,334; 3,944,335; 3,953,107; 4,052,121 and 4,342,502, which are incorporated by reference herein.
In U.S. Pat. No. 4,883,963 to Kemeny et al. and assigned to the assignee of the present invention, a birefringent AOTF is used in an in-line monitoring system for rapid analysis of a moving or changing sample. A variety of scanning patterns are shown.
In U.S. Pat. No. 4,602,342 to Gottlieb et al., an AOTF of mercurous chloride and related crystals is disclosed for use in an analysis system which utilizes one or two polarizers to isolate one tuned beam. In U.S. Pat. No. 4,663,961 to Nelson et al., optical fibers are used to carry radiation to and from a birefringent AOTF. This system also isolates one tuned beam through polarizers.
To determine a baseline, the Gottlieb and Nelson systems measure the response of an empty sample cell. Measurements of actual samples are then compared to the reference. It appears that only one reference measurement is used to correct all future sample tests. Since the reference measurement is not close in time to the sample measurements, noise and drift affecting the sample measurement may not be corrected. Fluctuations occurring after the reference test is run cannot be compensated for.